Thank you.
It turned out to be blocked with sooty deposits and once removed works fine.
DS
December 2014
Either the thermocouple that senses the heat of the pilot flame is defective - or - the Control electronics are defective - or the main gas valve is defective.
A small chance that the pilot flame is too weak to heat the thermocouple sufficiently - partially blocked with grit for instance.
When the gas-safe engineer repairs it for you - and if he claims that more than one part is defective - do not believe him.
It would be a most remarkable coincidence if two parts had failed at the same time.
Good luck...
December 2014
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